The present invention relates to the manufacture of microwave filters and it concerns in particular a system for synthesising discontinuity microwave filters for use in a satellite telecommunications network.
Microwave filters are commonly used in satellite pay-loads as constituent parts of output multiplexing networks. It is of particular importance in said application to avoid the passive intermodulation products that can seriously deteriorate the transmission performance. To avoid the passive intermodulation products it is necessary that the filter cells be calculated and implemented very accurately so that all tuning elements that can generate the passive intermodulation products are eliminated.
A microwave filter consists of a rectangular or circular waveguide in which discontinuities are formed to provide a plurality of cavities that are coupled to each other to allow the waves to propagate through the waveguide. Each cavity includes at least one tuning element serving to tune the resonant frequency of each cavity.
The development of a waveguide filter has always involved a significant experimental effort. After having defined the electrical characteristics of the filter to be designed, the equivalent electric network of the filter is determined. Thereafter, the geometrical dimensions of the cavities or resonators to be implemented in the filter are determined. The complete filter is then assembled and the electrical characteristics thereof are measured, and thereafter the tuning elements are adjusted manually in order to tune the measured electrical characteristics so that they comply with the desired characteristics.
In a microwave filter, however, the tuning of any resonator in the complete filter influences the electrical behavior of the preceding resonators as a result of interactions between the higher-order propagation modes. It is therefore necessary to measure the electrical characteristics of each coupling element after any individual tuning of a tuning element. A complete characterization of the coupling elements requires the measurements and adjustments to be repeated for each frequency in the filter bandwidth. Accordingly, the development of a microwave filter is a time-consuming task which requires considerable time for the tuning procedure when somewhat sophisticated filters are to be designed.
The electromagnetic analysis of the discontinuities in waveguides has been the subject matter of a number of publications which have proposed equivalent network representations for waveguide discontinuities using equivalent networks that are useful for deriving the computation of the physical elements. These known representations are divided into two groups. The representations of the first group reduces the discontinuity to a network and an integral equation that represents the effect of the higher-order propagation modes on the fundamental mode only of the waveguide (see e.g. N. Marcuvitz, Waveguide Handbook, McGraw-Hill Book Co., New York, 1951). The integral equation as proposed is a very efficient computation tool but the result thereby obtained is strongly limited in that it does not take into account the interactions between higher-order modes.
The known representations of the second group take into account the interactions between higher-order propagation modes. An example is the mode-matching method that is disclosed by T. Itoh in Numerical Technique for Microwave and Millimeter-Wave Passive Structures, John Wiley and Sons, Inc., New York, 1989. The drawback of this technique resides in that it results in a very slow convergence of the results and that it does not provide an explicit network form. In particular, most of the computation must be effected for each frequency and to ensure the proper convergence it is often necessary to explicitly consider a large number of modes thus resulting in very long computation run times that are generally not acceptable when optimization is required.
A novel formulation has been developed recently for deriving multi-mode equivalent networks for zero-thickness inductive discontinuities in a waveguide (M. Guglielmi and C. Newport, Rigourous, Multimode Equivalent Network Representation of Inductive Discontinuities, IEEE Transactions on Microwave Theory and Technique, Vol. 38, No. 11, November 1990). This particular formulation makes it possible to represent the coupling between the higher-order modes by a coupling matrix in form of impedances or admittances which are defined via an integral equation that is essentially independent from frequency and from the absolute dimensions. After the discontinuity has been characterized by solving the integral equation, the frequency dependence is introduced through a set of linear equations. This decomposition makes this approach numerically very efficient and very rapidly convergent. This solution is limited by the zero-thickness assumption for the discontinuity, however, wherein the discontinuity, either window or obstacle, is provided in a simple thin plate.
Now, the problem that claims the attention in designing sophisticated waveguide filters intended for use on satellites consists in designing waveguide filters including step discontinuities. To solve this problem, no practical solution was available heretofore, which overcomes the time-consuming manual tuning procedures or long run-time software optimization procedures.
The object of the present invention is to solve this problem and to that effect it provides an automatic system for synthesising a microwave filter with a rapid synthesis convergence, thus substantially reducing production time and cost.